1. Field of the Invention
The present invention is related to the field of video signal processing and, more particularly, to methods and apparatus for identifying whether a video signal originated by means of a film source or a video camera source.
2. Description of the Background
Film and television image generation systems give rise to the appearance to the viewer of continuously moving visual images. Actually, the appearance of continuous motion results from visual and mental integration by the viewer of rapidly advancing sequences of still frame images.
Conventionally, in countries having a 60 hertz power grid, motion picture films are generated and are projected at one frame rate, such as 24 film frames per second, while television images are generated and displayed at another frame rate, such as 30 television frames per second.
Each individual frame is made up of fields. The fields are typically produced in one of two ways. The fields may be interlaced, i.e. one field being made up of even numbered lines and the other field being made up of odd numbered lines, or the fields may be progressive, i.e. one line scanned after another. For example, in the standard NTSC format used for television, the 30 frames per second are comprised of 60 interlaced fields per second, or more precisely 59.94 fields per second within the NTSC color standard signal format.
A video sequence may contain a mixture of interlaced video and converted progressive film material. The film material is typically sampled at 24 hertz while video is sampled at different rates: 59.94 hertz for the NTSC standard, and 50 hertz for the D1-525 and D1-625 standards. A telecine, a device for scanning and converting film into video, must maintain a proper sequence duration between the different frame rates. To do so, either a 3/2 or 2/2 pull-down technique is used to stretch the display period of film frames to the target video standard. In the 3/2 pull-down process, one film frame is used to produce three video fields, and the next film frame is used to produce two fields, in a repeating 3/2 pattern. Thus, the 3/2 pull-down is used to convert two film frames into five video fields for the D1-525 video standard. In the 212 pull-down system, each film frame generates two video fields, one of each type (odd/even).
When receiving video information that was originally produced on film, there is an opportunity for performing essentially error-free de-interlacing of the signal. That is because each frame of the film source is used in generating at least two video fields, representing both types (odd/even) of interlaced fields. Therefore, if a video signal can be reliably determined to have originated on film, and the video fields corresponding to a common film frame can be identified, an essentially error-free non-interlaced video frame corresponding to a single instant in time can be generated by merging the two fields. Other uses of film source identification include identification of redundant fields (which occur in 3/2 pull-down sources) to be deleted in digital transmission systems for improving channel efficiency. Also, to display video in a progressive monitor, each field needs to be interpolated vertically to the full frame size. For the film-source video, the best way to de-interlace is to merge the two fields back together to reconstruct the original progressive film frame and discard any extra repeated field.
Unfortunately, no special information is included in broadcast or other video signals to indicate which fields may have originated on film and which fields may have originated in a video camera, so the presence of film-based material must be inferred by examining differences between the luminance information of fields. That, however, can present a number of problems. For example, a strong similarity between successive video fields could indicate that they were generated from the same film frame; it could also be due to a lack of movement in the program material. Likewise, a difference between fields may indicated that the fields did not come from the same frame of information, but the difference could also be due to vertical spatial detail or transmission noise. A practical film detector must therefor distinguish between the foregoing situations.
U.S. Pat. No. 5,689,301 discloses a method of film mode identification which comprises concurrently providing a first pixel from a given field and second and third vertically aligned pixels of the same horizontal position from a temporally adjacent field. The values of the pixels are compared to produce for each first pixel, a pixel difference signal having a value of zero if the value of the first pixel is intermediate the values of the second and third pixels. Otherwise, the difference signal has a value equal to the absolute value of the difference between the value of the first pixel and the value of one of the second and third pixels having a value closest to that of the first pixel. Non-zero values of the pixel difference signals are accumulated over a predetermined portion of one field period of the video signal to provide a field difference signal. The accumulated values are then analyzed for a pattern indicative of a film source of the video signal.
Despite advances in the field of identifying original film sources, the need exists for a practical detector which can correctly detect the occurrences of film sources in video sequences, which may contain an arbitrary mix of interlaced and progressive sources.